X-ray radiation applied to the interior of a patient's anatomical structure, for example to the soft tissue lining a body cavity of the patient, is known to be useful in the treatment of tumors. Diseases other than tumors can be treated in a similar manner, for example x-rays can be applied to the interior of blood vessels in order to prevent restenosis. In these and other treatments, most conventional x-ray therapy utilizes an external radiation source which directs relatively high energy x-rays toward the patient. The x-rays must first penetrate the skin and other tissue disposed between the x-ray radiation source and the target tissue, prior to reaching the tissue lining the body cavity. The exposure to such x-rays often causes significant damage to the skin and the tissue between the x-ray source and the target tissue.
Brachytherapy, on the other hand, is a form of treatment in which the source of radiation is located close to or in some cases within the area receiving treatment. The term brachytherapy has commonly been used to describe the use of radioactive “seeds,” i.e. encapsulated radioactive isotopes which can be placed directly within or adjacent the target tissue to be treated. Handling and disposal of such radioisotopes, however, may impose considerable hazards to both the handling personnel and the environment.
The term “x-ray brachytherapy” is defined in the present application as an x-ray radiation treatment in which the x-ray source is located close to or within the area receiving treatment. X-ray brachytherapy typically involves positioning an insertable probe into or adjacent to the tumor, or into the site where the tumor or a portion of the tumor was removed, to treat the tumor or the tissue adjacent the site with a local boost of radiation. X-ray brachytherapy devices generally include a miniaturized low power radiation source, which can be inserted into, and activated from within, a patient's body. In x-ray brachytherapy, therefore, x-rays can be applied to treat a predefined tissue volume without significantly affecting the tissue adjacent to the treated volume. Also, x-rays may be produced in predefined dose geometries disposed about a predetermined location. X-ray brachytherapy offers the advantages of brachytherapy, while avoiding the use and handling of radioisotopes. Also, x-ray brachytherapy allows the operator to control over time the dosage of the delivered x-ray radiation.
X-ray brachytherapy systems are disclosed, by way of example, in U.S. Pat. No. 5,153,900 issued to Nomikos et al. (“the '900 patent”), U.S. Pat. No. 5,369,679 to Sliski et al. (“the '679 patent”), U.S. Pat. No. 5,422,926 to Smith et al. (“the '926 patent”), and U.S. Pat. No. 5,428,658, to Oettinger et al. (“the '658 patent”), all of which are owned by the assignee of the present application, and all of which are hereby incorporated by reference in their entireties. The x-ray brachytherapy systems disclosed in the above-referenced patents include a miniaturized, insertable probe, which emits low power x-rays from a nominal “point” source located within or adjacent to the desired region to be affected. For example, the x-ray probe assembly disclosed in the '900 patent includes a housing, and a hollow, tubular probe extending from the housing and having an x-ray emitting target at its distal end. The probe encloses an electron source (such as a thermionic cathode) for generating electrons that are accelerated so as to strike the x-ray target. The x-ray brachytherapy device disclosed in the '658 patent includes a flexible x-ray probe, for example a flexible fiber optic cable enclosed within a metallic sheath, and uses a photocathode as the electron source. The flexible fiber optic cable couples light from a laser source or a light emitting device to the photocathode, which generates free electrons (due to the photoelectric effect) when irradiated by the light from the light source.
A number of patents describe x-ray brachytherapy systems which can produce x-rays in predefined dose geometries disposed about a predetermined location. U.S. Pat. No. 5,621,780 (hereinafter the “'780 patent”)(commonly owned by the assignee of the present application and hereby incorporated by reference in its entirety) discloses an apparatus and method for irradiating a surface defining a body cavity in accordance with a predetermined dose distribution. The '926 patent discloses an apparatus and method for irradiating a volume in accordance with a predetermined dose distribution. In particular, the '926 patent discloses a variable transmission shield which is adapted to control the position of the isodose surfaces of the x-rays emitted from an x-ray target element.
When thermionic cathodes are used in x-ray brachytherapy devices, it is desirable that the cathode be heated as efficiently as possible, namely that the thermionic cathode reach as high a temperature as possible using as little power as possible. In conventional thermionic cathodes, a filament is heated resistively with a current, which in turn heats the cathode so that electrons are generated by thermionic emission. These types of cathodes frequently encounter a number of problems, for example: 1) thermal vaporization of the cathode filament, resulting in tube failure; and 2) degradation in the x-ray output due to heating of the anode and resulting localized surface melting and pitting. While a photocathode avoids such problems, it is difficult to fabricate photocathodes in the vacuum.
The '568 patent discloses a miniature therapeutic radiation source that uses a laser-heated thermionic cathode, which overcomes the problems described in paragraph 6 above. The laser-heated thermionic cathode disclosed in the '568 patent provides a reduced-power, increased efficiency electron source for the x-ray source. The '568 patent discloses that by using laser energy to heat the electron emissive surface of a thermionic cathode, instead of resistively heating the cathode, electrons can be generated with minimal heat loss, and with significantly reduced power requirements.
Because of the advantages of x-ray brachytherapy, described in paragraph 3, it is desirable to use x-ray brachytherapy to treat the soft tissue that lines body cavities. It is also desirable to establish a uniform or other desired contoured dose of radiation to the target tissue, using x-ray brachytherapy devices. For this purpose, an x-ray brachytherapy system is needed which can be easily inserted into an interior body cavity, and can be easily controlled and maneuvered while in operation within the cavity. In some cases, it is desirable that radiation treatment of the tissue lining the interiors of a body cavity provides the same dose of radiation to every segment of the tissue, i.e. a uniform dose. In other cases, specifically contoured non-uniform doses may be desired.
For these reasons, it is desirable to provide a low power, miniaturized x-ray brachytherapy system, which is implantable within a body cavity of a patient or attached adjacent to a desired anatomical region of a patient, so that tissue forming the anatomical region or tissue lining the body cavity can be directly irradiated with x-rays. In particular, it is desirable to provide an implantable and easily controllable x-ray brachytherapy system that can use an optically activated electron source, because of the associated advantages set forth in paragraph 7. It is further desirable that such a miniaturized x-ray brachytherapy system be operable to irradiate a selected volume of a desired anatomical region, and to establish an absorption profile defined by predetermined isodose contours. It is further desirable that the miniaturized x-ray brachytherapy device be operable to provide a uniform, or other desired, dose of x-ray radiation to the tissue that lines a body cavity.